Baker/Alpha: distinguish the consensus key from the delegate key

What

Distinguish the consensus key from the delegate key more clearly (using the type system).

On top of !16289 (merged).

Why

The fact that the consensus key can be easily confused with the delegate key has bitten us twice: see #7648 (closed) and !16289 (merged).

How

Introduce abstract types for these two kinds of keys.

Also do some further renamings. For instance, refer to the pkh as the id of a key.

(It may be useful to make a similar distinction in the protocol code.)

To the reviewers: the MR consists of a sequence of refactoring in a somewhat arbitrary order.

Manually testing the MR

The CI should be sufficient?

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR